1. Field of the Invention
This invention relates to a stimulable phosphor panel. This invention particularly relates to a stimulable phosphor panel provided with a water vapor proof cover, which seals a stimulable phosphor layer.
2. Description of the Related Art
Radiation image recording and reproducing systems utilizing stimulable phosphors have heretofore been known as computed radiography (CR). The radiation image recording and reproducing systems utilizing the stimulable phosphors comprise a radiation image recording apparatus for irradiating radiation carrying image information of an object, such as a human body, and storing a radiation image of the object as a latent image on a layer of the stimulable phosphor. The radiation image recording and reproducing systems utilizing the stimulable phosphors also comprise a radiation image read-out apparatus for exposing the stimulable phosphor layer, on which the latent image of the radiation image has been stored, to stimulating rays, such as a laser beam, which cause the stimulable phosphor layer to emit light in proportion to the amount of energy stored on the stimulable phosphor layer during the exposure of the stimulable phosphor layer to the radiation, and photoelectrically detecting the emitted light in order to acquire an image signal representing the radiation image of the object.
As recording media utilized in the aforesaid radiation image recording and reproducing systems, stimulable phosphor panels comprising a substrate having rigidity, a stimulable phosphor layer, and a protective layer, which are overlaid one upon another in a layer form, have been known. After image signals have been detected from the stimulable phosphor panels, the stimulable phosphor panels are exposed to erasing light, and energy remaining on the stimulable phosphor layers is thus erased. The erased stimulable phosphor panels are then capable of being used again for the recording of radiation images. In this manner, the stimulable phosphor panels are capable of being used repeatedly for the recording and readout of radiation images.
Also, the stimulable phosphor panels are used for a plurality of times of radiation image recording and read-out operations over a long period of time. Therefore, it is necessary that the stimulable phosphor panels be protected from deterioration due to external physical and chemical attacks. In particular, certain kinds of stimulable phosphors constituting the stimulable phosphor layers are apt to absorb moisture, and the radiation image recording and read-out performance becomes bad markedly due to the moisture absorption of the stimulable phosphors. Therefore, in order for such problems to be eliminated, a stimulable phosphor panel has heretofore been proposed, wherein a transparent material having water vapor proof characteristics is utilized as a transparent water vapor proof cover acting as the protective layer, and the stimulable phosphor layer is enclosed and sealed between the transparent water vapor proof cover and the substrate.
However, in cases where the stimulable phosphor panel described above, wherein the stimulable phosphor layer is enclosed and sealed between the transparent water vapor proof cover and the substrate, is used at the high ground, at which the atmospheric pressure is low, or under environmental conditions of high temperatures, the problems often occur in that a gas contained in an enclosed region between the transparent water vapor proof cover and the substrate expands and causes an increase in a spacing between adjacent layers or distortion of the stimulable phosphor layer to occur.
If the spacing between the transparent water vapor proof cover and the stimulable phosphor layer becomes large, the problems will occur in that, when the stimulating rays are irradiated to the stimulable phosphor layer via the transparent water vapor proof cover during the readout of the radiation image from the stimulable phosphor layer, on which the radiation image has been stored, the stimulating rays are reflected repeatedly between the stimulable phosphor layer and the transparent water vapor proof cover, and the exposure area of the stimulable phosphor layer exposed to the stimulating rays becomes wide.
FIG. 11 is an explanatory side sectional view showing how stimulating rays spread due to repeated reflection of the stimulating rays in cases where a spacing between the stimulable phosphor layer and a water vapor proof film becomes large. Specifically, as illustrated in FIG. 11, in cases where the spacing between a stimulable phosphor layer 20 and a water vapor proof film 30 acting as the transparent water vapor proof cover becomes large, and a space 41 A has thus occurred between the stimulable phosphor layer 20 and the water vapor proof film 30, a distance of propagation of stimulating rays Le due to one time of reflection between the water vapor proof film 30 and the stimulable phosphor layer 20 becomes long, and therefore the stimulating rays Le having been reflected between the water vapor proof film 30 and the stimulable phosphor layer 20 spreads over a range of an area R1. FIG. 3 is an explanatory side sectional view showing how stimulating rays spread due to repeated reflection of the stimulating rays in cases where a spacing between a stimulable phosphor layer and a water vapor proof film has not become large, in which view, as an aid in facilitating the explanation, a close contact interfacial boundary between the stimulable phosphor layer and the water vapor proof film is illustrated as if the boundary had a certain thickness. As illustrated in FIG. 3, in cases where the spacing between the stimulable phosphor layer 20 and the water vapor proof film 30 acting as the transparent water vapor proof cover are in close contact with each other or close to each other, the distance of propagation of the stimulating rays Le due to one time of reflection between the water vapor proof film 30 and the stimulable phosphor layer 20 is short, and therefore the stimulating rays Le having been reflected between the water vapor proof film 30 and the stimulable phosphor layer 20 spreads over a range of an area R2, which is narrower than the area R1 described above. Accordingly, in cases where the stimulating rays Le are reflected the same times between the water vapor proof film 30 and the stimulable phosphor layer 20, the exposure area of the stimulable phosphor layer 20, which is exposed to the stimulating rays Le in cases where the spacing between the stimulable phosphor layer 20 and the water vapor proof film 30 is large, becomes wider than the exposure area of the stimulable phosphor layer 20, which is exposed to the stimulating rays Le in cases where the spacing between the stimulable phosphor layer 20 and the water vapor proof film 30 is small.
Therefore, in the cases of FIG. 11, the light is emitted by the stimulable phosphor layer 20 with the effect identical with the effect occurring when the stimulable phosphor layer 20 is exposed to the stimulating rays having a large beam diameter corresponding to the wide area R1. As a result, the problems occur in that the light emitted from the wide area R1 of the stimulable phosphor layer 20 containing an area other than a predetermined detection range is detected, and the image sharpness of the radiation image represented by the thus acquired image signal becomes low.
FIG. 14 is an explanatory side sectional view showing how a radiation image is read out from a distorted stimulable phosphor layer. As illustrated in FIG. 14, in cases where the stimulable phosphor layer has been distorted due to expansion of the gas contained in the enclosed region, when the stimulating rays Le are irradiated to the stimulable phosphor layer 20, the stimulating rays Le impinge upon a position H2 on the distorted stimulable phosphor layer 20, the position H2 being shifted from a predetermined position H1, upon which the stimulating rays Le impinge in cases where the stimulable phosphor layer 20 has not been distorted. Also, the stimulating rays Le, which are to be irradiated to the predetermined position H1 on the stimulable phosphor layer 20 having not been distorted (i.e., a position H1′ on the distorted stimulable phosphor layer 20), impinge upon the position H2 on the distorted stimulable phosphor layer 20, which position H2 is shifted from the predetermined position H1, and the light emitted from the position H2 is detected by detecting means 90, which has been set so as to detect the light emitted from the predetermined position H1. As a result, the efficiency, with which the emitted light is collected by the detecting means 90, becomes low. Further, since the light emission position alters to the position H2 shifted from the predetermined position H1′(the predetermined position H1) on the stimulable phosphor layer 20, the radiation image represented by the image signal acquired from the shifted position H2 becomes a distorted image. Accordingly, the radiation image having been stored on the stimulable phosphor layer 20 cannot be read out accurately.